Download Chapter 35 Plant Structure

Biology 120
J. Greg Doheny
Chapter 35
Plant Structure
Plants have three basic organs (roots, stems, and leaves), and three basic tissues (dermal tissue,
vascular tissue, and ground tissue). The basic function of roots is to anchor the plant in the
ground, and extract water (used in photosynthesis) from the ground. The basic function of the
stem is to elevate the leaves as high above the ground as possible, allowing the plant to compete
for light. The basic function of leaves is to carry out photosynthesis (including transpiration).
The basic function of dermal tissue is to prevent excessive water loss from the plant (beyond
what is lost in transpiration), and protect the plant from herbivores (mainly insects). The basic
function of the vascular tissue is to transport water up from the roots to the leaves (where it is
needed for photosynthesis), and transport the sugars created by photosynthesis from he leaves to
other parts of the plant. The basic function of ground tissue is to carry out plant metabolism.
Various adaptations and modifications of these tissues also exist.
TYPES OF PLANTS
Vascular vs. Non-Vascular Plants: There are two basic types of photosynthetic land plants.
Those that do not have vascular tissue to carry water and nutrients around, and those that do.
Those that do not have vascular tissue (ie-mosses) cannot grow very tall, because the water can
only get from the root to the photosynthetic tissue by diffusion. Vascular plants can actually
transport water from the ground to the leaves in tubes, and are able to grow much taller.
Growing taller also requires vascular plants to have support tissue. Two polysaccharides,
cellulose and lignin are the main support tissues in vascular plants.
Gymnosperms vs. Angiosperms: There are two main types of vascular plants, the
gymnosperms and the angiosperms. The gymnosperms produce seeds that are ‘naked’ (are not
surrounded by an ovary). Gymnosperm seeds are generally dispersed by wind. (Examples: pine
trees etc.) Angiosperm seeds are contained in a structure called an ovary, that contains several
seeds, and (usually) some ‘fruit.’ The fruit is often attractive food for herbivores who then help
to disperse the seeds. (Examples: fruit trees, flowers, nut trees etc.) Angiosperms also tend to
produce flowering reproductive structures (ie-flowers) which attract insects (ie-bees) who help
pollinate the plants.
Angiosperms (Monocots vs. Eudicots): Angiosperms are classified into two basic types, the
monocots and the eudicots, based on morphological characteristics as follows:
1. Cotyledons: A cotyledon is a ready-made leaf that is contained in the seed. These leaves
are the first to emerge when the seed germinates. Monocots have one cotyledon, eudicots
have two.
2. Leaf Veins: Are parallel in monocots and branched in eudicots.
3. Vascular tissue: Vessels are scattered throughout the stem in monocots, but organized
into rings in eudicots.
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4. Roots: Monocots have fibrous roots, where individual roots branch (bifurcate) into two
roots at many points. Eudicots have a single, large root called a taproot, that may have
lateral branches.
5. Pollen: Monocot pollen has only one opening, while eudicot pollen has three or more
openings.
6. Flowers: Monocot flower petals are organized in multiples of three (3, 6, 9 etc.), while
eudicot petals are organized in multiples of fours or fives (4, 8 etc., or 5, 10 etc.).
PLANT ORGANS
ROOT SYSTEMS (Figure 35.2 and 35.4): Roots anchor the plant in the ground, and absorb
water from the ground. Plants can have either a taproot system (eudicots), with one large, main
root, and smaller, lateral roots attached; or a fibrous root system (small, thin, bifurcating roots
that stay near the surface). Some modified taproots store carbohydrates (ie-beets). Fibrous root
systems generally do not penetrate deep into the soil. This is useful when the soil is shallow (ieif there is a layer of rock or clay underneath). An extensive fibrous root system will prevent a
plant from being blown away if it is rooted in shallow soil. Also, if water is scarce, a wide,
lateral fibrous root system can gather rainwater more efficiently than a deep root system. Some
roots have ‘root hairs,’ which are not actually composed of individual cells (Figure 35.3).
Instead, they are small tubular extensions of the epidermal cells surrounding the root. Their
purpose is also to help extract water from the soil.
Variations and Types of Roots (Figure 35.4):
1. Radicle: the first root to emerge from a plant seed.
2. Taproot: one large, main root with lateral branches.
3. Storage Roots: (usually taproots) Roots specially adapted to store starch. (ie-beets,
carrots)
4. Prop Roots: (Also known as plank roots or buttress roots) large roots that are reinforced
with cellulose and lignin, which extend above ground, and help support the tree.
(Example: the Ceiba tree, which grows in the Amazon basin.) A buttress root is
sometimes called a ‘plank buttress’ because it looks like a wooden plank that was nailed
to the tree to ‘buttress’ (to hold up) the tree. Prop roots are used to help stabilize large,
top-heavy trees in areas where the ground is shallow and underlain by rock or water.
5. Aerial Roots: (Also known as Strangling aerial roots) Aerial roots are produced by
epiphytes (plants that grow on top of other plants) that are growing atop other trees. The
roots hang down from the host tree like snakes, until they reach and take root in the
ground. Aerial roots also grow down the trunk of the host tree, and appear to strangle it.
6. Pneumatophores: Roots produced by trees that grow in swamps or tidal estuaries. (iemangrove trees) The tips of the roots stick up above the ground, allowing the roots to
obtain oxygen needed for aerobic respiration. (Remember that almost all types of
metabolically active cells need to be able to ‘breath,’ because their mitochondria need
oxygen to carry out aerobic respiration. This means that plants can usually only grow in
‘aerated soil.’ ie-soil that allows air to diffuse into it.)
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STEM SYSTEMS: The purpose of stems is to raise the photosynthetic leaves as high into the
air as possible, so the plant can compete for sunlight. Thus, stems tend to be reinforced with lots
of support material (lots of cellulose and lignin). Stems also contain the ‘buds’ from which
leaves arise. Several buds, which give rise to several ‘branches’ of a plant tend to occur in
bunches called ‘nodes.’ Thus, several branches, or several leaves appear to arise from the same
place (a node) on a stem. The areas in between nodes are called ‘internodes’ (Figure 35.2). The
buds from which the leaves or branches arise are labelled as either apical buds or axillary buds,
depending on whether they are at the end of a shoot (stem) or not. Apical buds are located at the
end of a shoot, and point in the same direction as the growth of the shoot. There is only one
apical bud per shoot. Axillary buds are located below the apical bud, and point in a direction
perpendicular to the apical bud. ‘Apical dominance’ describes the phenomenon whereby only
the apical bud will grow into a leaf, set of leaves, or another shoot; and the axillary buds will
remain dormant. However, if you cut off the apical bud, one of the dormant axillary buds will
then become the apical bud by default, and start to grow.
Types of Modified Stems (Figure 35.5): The following are modifications or adaptations of
stems.
1. Rhizome: A fleshy part of the stem that extends underground, horizontally. Rhizomes
are often mistaken for roots.
2. Bulb: Vertical underground shoots/stems made of the enlarged bases of fattened storage
leaves. (Leaves that are modified to contain starch. Examples: Onions, tulip bulbs.)
3. Tuber: An ellipsoid-shaped (round or oval) modification of a stem that stores starch, is
found underground, and often mistaken for a root (ie-potatoes).
4. Stolon: A stem that grows horizontally, just above or below the ground, giving rise to
new plant nodules next to the existing ones. (Sometimes called ‘runners.’)
LEAF SYSTEMS: The leaf is the main photosynthetic organ of the plant, although stems often
contain chloroplasts, and are capable of photosynthesis. Leaves are classified as the main
photosynthetic organ because they have a larger surface area, and can thus carry out more
photosynthesis than the stems of a plant. Each leaf has a thick section, called a petiole, where it
makes contact with the stem. The petiole is always associated with an axillary bud. Several
‘veins’ then supply the leaf with water, and return sugars to the rest of the plant. As mentioned
above, monocot leaf veins are parallel, and eudicot leaf veins are branched. A leaf will then have
one or more ‘blades’ which collect the sunlight. Each leaf may have one or more ‘lobes’ (round
bulges).
Variations and Types of leaves (Figure 35.6): Leaves are classified as simple, compound, or
doubly compound as follows.
1. Simple leaf: Has a single, undivided blade with one or more lobes.
2. Compound leaf: The petiole branches out into several ‘leaflets’ instead of a single leaf.
The way to tell the difference between a compound leaf and a branching stem is that the
individual leaflets do not have an axillary bud associated with them.
3. Doubly compound leaf: The petiole spreads out into several more petioles, each of
which spread out into multiple leaflets. Again, these are classified as compound leaves,
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rather than branching shoots or branching stems because of the absence of buds
associated with each branch.
Leaves Modified for Special Purposes (Figure 35.7): Leaves can have several adaptations
which help their parent plants survive.
1. Tendrils: A tendril is a leaf that reaches out and wraps itself around a support, to help the
plant grow upwards. (ie-beans, grape vines, morning glory)
2. Spines: Some plants have developed spines or spikes on their leaves to discourage
herbivores from eating them. (ie-cacti)
3. Storage leaves: Some plants have leaves that are modified to store starch (ie-onions)
4. Reproductive leaves: Some (rare) plants produce leaves that are actually tiny offspring
plants that fall off and take root.
5. Bracts: Highly coloured leaves that are located near the flowering structures of plants,
and are used to attract insects (ie-bees) to help with pollination. They are often mistaken
for flowers, but they are not part of the flowering reproductive structure. (ie- Poinsettias)
PLANT TISSUES (Figure 35.8)
As mentioned above, plants have three basic types of tissues: Dermal tissue, which maintains the
integrity of the plant (discourages herbivores, prevents water loss); vascular tissue which
transports water and sugars throughout the plant, and ground tissue, which is composed of highly
metabolically active cells.
Dermal Tissue: The epidermis is the outermost layer of tissue. The epidermis is usually coated
with a waxy cuticle, which prevents water loss, and is difficult for insects to bite through. The
epidermal layer of some plants is sometimes covered with hair-like structures called trichomes
that also discourage insects.
Vascular Tissue: There are two types of vascular tissue, Xylem and Phloem.
1. Xylem: conducts water upward from the roots. Is composed mostly of the cell walls of
dead cells, and acts like a ‘water wick,’ where water is drawn up from the ground to
replace the water that is being lost from transpiration.
2. Phloem: live tissue that transports the sugars produced by the Calvin Cycle to other parts
of the plant.
Ground Tissue: There are two basic types of ground tissue. Ground tissue located within, or in
between the vascular tissue is called pith, and ground tissue located outside (and usually around)
the vascular tissue is called cortex. Cortical tissue is divided into three types:
1. Parenchyma: Parenchymal tissue is composed of cells that are often metabolically
active, and have very thin cell walls. Starch is stored in parenchymal cells. Parenchymal
tissue provides the soft ‘filler’ tissue of many plants, including the mesophyll cells of
leaves. (Note: in human tissues and organs, the metabolically active tissue which carries
out the functions of the organ is referred to as the parenchymal tissue, as opposed to the
structural tissue of an organ, which is called the stroma.)
2. Collenchyma: Less metabolically active than parenchymal cells, organized into thin
strands, used to support and supply growing shoots and stems. The cells are often
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elongated into an ellipse. Collenchymal cells are fortified by thicker cell walls than
parenchymal cells, and provide structural support for newly grown plant tissue.
3. Sclerenchyma: Less metabolically active than collenchymal cells. Reinforced with more
lignin and cellulose, and have thicker walls. Sclerenchymal cells provide the main
structural support for plants. Sclerenchymal tissue is sometimes organized into small,
knot-like bundles in fruits called sclereids. It is the presence of sclereids in fruits that
give fruits like pears their ‘gritty’ texture.
PRACTICE QUESTIONS:
Short Answer Questions:
1. What are sclereids, where would you find them, and what are they composed of? (2
points)
2. What are the two basic functions of roots? (2 points)
3. What are the two main polysaccharides that make up support tissue in vascular plants? (2
points)
4. What is the main purpose of a plant stem?
5. What is the main purpose of plant leaves?
6. What is the main purpose of dermal tissue in leaves?
7. What is the main purpose of ground tissue?
8. What type of tissue gives rise to the root hairs seen on some roots?
9. Give one example of a storage root that you can buy at the grocery store.
10. List three differences between a monocot and a eudicot. (3 points)
11. Is a potato a modified root, modified leaf, or modified stem? What is the technical name
for this modified structure? (2 points)
12. Is a beet a modified root, modified stem, or modified leaf? What is the technical name
for this modified structure? (2 points)
13. Is an onion a modified root, modified stem, or modified leaf? What is the technical name
for this modified structure? (2 points)
14. Is the tendril of a bean plant a modified root, modified stem, or modified leaf?
15. What is the name for the most metabolically active type of tissue that makes up ground
tissue. (Hint: the metabolically active tissues that make up human organs have the same
name.)
16. What is the name of metabolically active ground tissue that is located within, or in
between vascular tissue in plants?
Description Questions:
1.
2.
3.
4.
5.
6.
What is a root radicle?
What is a cotyledon?
What is a ‘plank buttress?’
What is a storage root?
What is a pneumatophore?
What is a tendril?
Short essay questions:
1. What is the difference between an axillary bud and an apical bud? (5 points)
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2. What is meant by ‘apical dominance’ with respect to plant buds? (10 points)
3. How can you tell the difference between the petiole of a compound leaf, and a branched
stem? (10 points)
4. What advantages to fibrous root systems have over taproot systems? (10 points)
Extended Matching: Match the term to the definition.
A.
B.
C.
D.
E.
F.
G.
H.
I.
J.
K.
L.
M.
Apical bud
Axillary bud
Bulb
Bract
Buttress
Cotyledon
Cuticle
Epiphyte
Internode
Lobe
Node
Ovary
Parenchymal
N.
O.
P.
Q.
R.
S.
T.
U.
V.
W.
X.
Y.
Z.
Petiole
Phoem
Pith
Pneumatophore
Pneumo/pneumatic
Radicle
Rhizome
Stolon
Taproot
Tendril
Trichome
Tuber
Xylem
1.
2.
3.
4.
5.
6.
An angiosperm seed container that contains several seeds.
The part of a leaf that makes contact with the stem.
Metabolically active ground tissue that is located in or between vascular tissue.
The rounded border of a leaf.
A prefix referring to ‘air.’
A waxy coating that covers the outside of a plant; preventing water loss, and
discouraging insects.
7. Term referring to the most metabolically active cells in ground tissue.
8. Term for dead tissue that transports water up from the roots of a plant.
9. A hair-like fiber found on the epidermis of plants; that discourages insects from eating it.
10. A modified leaf that wraps around a support. (Used by plants that don’t have very strong
support stems. Example: beans)
11. Term for live tissue that transports the sugar products from photosynthesis around the
plant.
12. A colourful leaf located near the flower, but which is not actually part of the flower.
13. An area on the stem of a plant that has no leaves or shoots arising from it.
14. An area on the stem of a plant where several leaves or shoots come out.
15. A modified stem structure that is often mistaken for a root. I stores starch, and is usually
round or ellipsoid in shape.
16. A bud located at the end of a stem or shoot, which points in the same direction as the
growth of the stem or shoot.
17. A thin stem that extends horizontally across (and above) the ground.
18. A modified stem structure made of the widened bases of storage leaves.
19. The first root to emerge from a plant seed.
20. A bud that points in a direction perpendicular to the growth of a stem or shoot.
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21. An underground stem or shoot that extends horizontally underground, and is often
mistaken for a root.
22. A large, central root, which sometimes has thinner, lateral branches.
23. A plant that grows on top of another plant.
24. A word meaning to prop up. Sometimes used to describe a prop root.
25. A root whose tips extend upwards out of the ground. Used by trees that grow in
waterlogged soil (ie-in swamps)
26. A plant leaf that is present in a plant seed.
© J. Greg Doheny 2014
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